Sinusoidal modulator



J: w. BOZEMAN SINUSOIDAL MODULATOR July 13, 1948.

Z'Sheets-Shee; 1

Filed April 10, 1947 INVENTOR. Jqlm hfflazeman, W

flTTOR/VEY y 1948- J. w. BOZEMAN SINUSOIDAL MODULATOR Filed April 10, 1947 INVENTOR. Jojm h I 17oz aman, M

- HTTORNEY Patented July 13, 1948 John W. Bozeman,

Control Instrument Company,

New York, N. Y. assignor to ho, Brooklyn,

N. Y., a corporation of'New York Application April 10, 1947}, Serial No. 740,649

This invention relates to improvements in the means for modulating the amplitude of alternating current voltages in a manner which will allow the modulated voltage to retain its sinusoidal wave-form without distortion. Ordinarily, amplitude modulation is accomplished by an element acting as a switch, which, being operated by the modulating voltage and operating up n the carrier voltage, converts the constant amplitude of the carrier voltage to a voltage whose amplitude varies at a predetermined rate.

In most applications, it is relatively unimportant as to whether or not the carrier is distorted in the process of being modulated, since the intelligence is contained only in the modulation frequency and demodulation will yield the intelligence back again; however, in applications where the carrier frequency is to be used as modulated, clipping, introduction of harmonics, or any other form of distortion caused by the switch action of, the modulator upon the carrier may give rise to undesirable effects, hence the need for a modulator which will permit the carrier to retain its undistorted sinusoidal wave-form.

Accordingly, this invention provides a means whereby a sinusoidal alternating voltage of one frequency may be made to vary its amplitude at the rate of a second, lower frequency, voltage, and retain its sinusoidal character. Furthermore, the means provided is such, that the amplitude of the output voltage, as modulated, is a linear function of the amplitude of the modulating voltage.

' These and other features of the invention will become apparent in the course of the following description, it being understood, however, that the description, specification and drawings are intended to illustrate rather than define and limit the scope of the invention, reference being made to the appended claims for that purpose.

In the drawings:

Fig. 1 is a diagrammatic the sinusoidal modulator;

Fig. 2 is a diagrammatic view of another form of the sinusoidal modulator;

Fig. 3 is a perspective view of the transformer therein employed;

Fig. 4 is a transverse section on the line 4-4 of Fig. 3;

Fig. 5 is a plot of the amplitude versus time of a voltage used as a carrier;

Fig. 61s a plot of the amplitude versus time of a voltage used as a modulating frequency; and

Fig. '7 is a plot of the amplitude versus time of the resultant modulated carrier.

Referring to the drawings, a sinusoidal voltage:

view of one form of V 11 Claims. (Cl. 179-r-171-5) E1 as shown in Fig. 5 is to be modulated with a voltage E2 as shown in Fig. 6, the resultant modulated voltage to retain the sinusoidal character of the initial voltage, as shown in Fig. '7.

The action of the modulator may be analyzed from the behavior of the elements shown in Fig. 1. The modulator consists of a bridge having as ratio arms, elements I and 2, a comparison arm element 3 and an unknown arm element 4. If an alternatingvoltage is applied between terminals 5 and 6, no voltage will appear at the terminals land 8 when the resistance of arm I equals the resistance of arm 2 and the resistance of arm 3' equals the resistance of arm 4. If the resistance of three of the four arms is made identical but the resistance of the fourth arm is made larger, a voltage will appear between terminals 1 and 8 having a magnitude determined by and proportional to the degree of unbalance, and a phase corresponding to the phase of the voltage between terminals 5 and 6. If the fourth arm is made smaller, a voltage will appear between terminals 1 and 8 having a magnitude determined by and proportional to the degree of unbalance, and a phase opposed to the voltage between terminals 5 and 6.

The vacuum tube 9 which constitutes the fourth bridge arm is not a linear resistor at best, and must be operated with proper electrode potentials in order that it behave like a resistor and not like a non-linear conductor or rectifier. To that end, a source of direct current 10, an isolation choke l l to prevent the alternating current in the bridge from having a shunt path other than the bridge proper, a bias resistor 12 to set the operating point of the tube, and a by-pass capacitor l3 to eliminate the effect of resistor l2 on the alternating current circuit, are all employed to make the use of the vacuum tube as a resistor a possibility.

Since the comparison arm of the bridge is a similar vacuum tube liand not a resistor, similar methods must be employed to make its use permissible. Therefore, it, too, is supplied from the direct current source 9 through the isolation choke l0, and furnished with a bias resistor l5 and a by-pass condenser l6. Its grid l1, however, is grounded so that its resistance may remain fixed.

The potential of the direct current source must be maintained higher than the peak carrier voltage applied to terminals 5 and 6. The plate voltage of the vacuum tubes as applied to the anodes 9 and M will be the instantaneous sum of the battery voltage, plus the applied carrier voltage which is applied to the terminals '5 and 6, and

will never be negative if the negative peak of the carrier does not exceed the battery voltage; thus the plate resistance of the vacuum tubes which is in part dependent upon plate voltage, will always be some real value, not infinity (which it would become if the plate voltage were permitted to become negative). The two tubes l6 and II will always-have comparable plate resistances and will function as resistors in the bridge circuit.

By changing the potential of the grid I8 of tube 9 by applying the modulating voltage E2 (Fig. 6), we may change its effective resistance in the bridge circuit. This grid is negative with respect to cathode [9 by virtue of the direct current 1. 3. drop in the bias resistor l2 "and, as long as the voltage applied to grid I 8 does not exceed the bias potential, the tube will operate Class-A,-will' draw no grid current, and will have the effect of a linear change in the plate resistance of vacuum tube 9. If the grid is made more positive but does not exceedithe bias voltage of the vacuum tube 9, the plate-resistance thereof will go down, thus unbalancing the bridge, and a voltage will appear across terminals land Band will increase in magnitude and have a phase corresponding to the voltage across terminals 5 and 6 and be proporti-onalt'o the amplitude of the modulating voltage E2. If the voltage is mademore negative but does not exceed the value of plate current cutoff for the vacuum tube 9, the plate resistance will go up, tin-balancing the bridge and a voltage will appear across terminals 1 and 8 and will have a phase opposed to the voltage across terminals 5 and 6 and be proportional to the magnitudeof the modulating voltage E2.

The output voltage, however, has its origin in the carrier voltage and no factors enter to distort'its wave-form. Its magnitude and phase, however, are now determined by the applied grid voltage'and the carrier is, therefore, modulated with complete linearity and without distortion.

Another form of themvention is shown in Fig. 2. This application consists of twocircuits such as are shown in Fig. l and hereinbe'fore described with resistors -l and Z'be'ing omitted from both and in their place inductances substituted therefor, combined-into a single transformer, with the output circuit being inductively coupled thereto, as shown by transformer 2-9.

As shown in Fig. 2, the carrier voltage E1 is introduced into transformer 2| across the terminals'ZUa and 23b thereof. The voltage appear ing at the secondary 22 of transformer 2| will consist of'two voltages as measured between terminals 23 and 24 and between terminals 23 and 25, which voltages are out of phase. Terminal 23 is maintained at ground potential for alternating current by means of capacitor 26. The voltage between terminals 23 and 24 of transformer 2| is applied to center tap 21 of winding 28 of transformer 29; the voltage between terminals 23 and 25 of transformer '2| is applied to center tap 30 of Winding 3| of transformer 29. Thus the terminals 32 .and 33 are at the same alternating potential, and terminals 34 and 35 are at the same alterating potential but opposite in phase to terminals 32 and'33.

By means of transformer 35, rectifier tube '37 and the filter network 38, a direct currentvoltage is made available and applied to "terminal 23 of transformer ill. The circuits will functionwithout this direct current voltage and it is not essential to the operation of the modulator unless an extended linear range is required. Elements 36, 3'! and-38 may, therefore, be-completely'omitted. However, they are shown in the drawing and included in the specification because of the extended range of operation permitted by their inclusion. The current flow due to this direct current voltage divides and flows in opposite directions, that is to say, from terminals 23 to 24 and from terminals 23 to 25 so that any compo nents due to this current flow has no effect on the alternating current characteristic of the circuit due to saturation of the transformer core.

Terminals 24 and 25 of transformer 2| are at the same direct current potential with respect to ground and because of the connections between terminals 24 and 21 and between terminals 25 and 3t], terminals 32, 33, 34 and 35 are at the same direct current potential with respect to ground. Terminal 32 is connected to anode 39 of triode 49 of the twin triodes comprising vacuum tube M and terminal 33 is connected to anode 42 of triode 43 of vacuum tube 40 (a twin triode being shown for convenience and separatetubes of any nature comprising three or more elements being applicable) thus energizing the two triode section anodes with similar direct current potentials. The cathodes 44 and 45 of said triodes are connected together through a resistor 46 and returned to ground through a variable tap 4i thereon in series with resistor 48 so that thedirect current in the two sections of the tube'ma'y be made identical. In the same way, terminal 35 is connected to anode '43 of triode 50 of vacuum tube 5|, terminal 34 is connected to anode 52 of triode 53 of vacuum tube 5|, thus-energizing both triode section anodes of vacuum tube 5| with similar direct current potentials. The cath- 0des-54 and 55 of said triodes are conne'cte'dtogether through resistor 55 and returned to ground through variable tap 51 thereon in series with resistor 48 so that the current in these-two tubes may be made identical. Resistor is-connecte'd in circuit so that the I. R. drop of the directcurrent therein affords the bias voltage neoessary'to set the operating point of all of the triodes. Grids 58 and 59 of vacuum tubes 40 and 50 are connected directly to ground. The remaining grids 68 and 5| are connected in parallel with resistor 32, which is included to provide a directcurrent return if the driving source should fail to provide one, and are connected to the source of the modulating voltage E2 (Fig. 6). 'When the modulating voltage E2 is zero all four triodes 43,4350 and 53 draw equal plate currents and sincethe plate currents for vacuum tube 4| flows from terminal 30 13032 and terminal 3!] to 33 in winding5| of transformer 25, that is to say, in opposite directions from the center tap thereof, and the plate currents for vacuum tube 5| flows from terminal 2'! to 34 and from Elto 35 in winding 28-of transformer 29, that is to say, in opposite directions from the center tap thereof, the direct current components cancel and produce no effectin the transformer.

When a carrier volta e E1 such as isshown in Fig. 5 is impressed between terminals 20a and 20b of transformer 2|, the potentials 'asthey-exist between terminals 23 and 24, and between 23 and 25, change so that their instantaneous value is equal to the direct current voltage plus the alternating current voltage. This voltage is never permitted to become negative, but varies about some positive point at the carrier frequency.

Because of the connections heretofore outlined for the direct currents, theano'de voltages will vary in the same way, the anodes 39 and'4'2'being made more positive while the anodes 49 and '52 tive, the gridslill and SI of sections rent voltage at terminal 30 arebeing'made more negative, and on, the re- I Theindividual tube currents, however, still balance in windings 28 and ,3! of transformer 29 and no voltage will therefore be induced in winding 63 thereof. 7 c r If .a modulating voltage E2 vsuch as shown in Fig. 6 is introduced across resistor 51 43 and 53 of vacuum tubes 4| and 5| are caused to go positive. The triodes 43 and 53 will. therefore draw larger plate currents, thus unbalancing the division of currents at terminal 21 and 30, larger currentsflowing from 21 to 34 and from 33 to 33 and smaller currents flowing from 28 to 35 and from 3| 0032. The magnetic effect of the direct current flow will continue'to be zero and cause no saturation because them'agneti-c effect of the current flow from 30 to 33 opposes the magnetic effect of the current flow from 21 to 34, and the magnetic effect of the current flow from 30 to 32 opposesthe magnetic effect of the current flow from 21 to 35. The magnetic effect of the alternating currents, however, will not be zero but will induce a voltage in winding 53 of transformer 29. When the alternating curis positive the current in the direction from 30 to 33 will exceed the current in the direction from 21 to 34 and on the remaining'half cycle when the alternating current 'voltageat 21 is positive, the current will flow in the direction from 21 to 34 and will'ex- 'ceed the current flow from 30 to 33 thus causing the alternating current to flow first from 30 to 33 and then'from 211to 34," which currents will 'inducea voltage in winding '63 whose magnitude is determined by the'voltageimpressed upon resistor 62 the grids EU'and 6| are'made positive. The voltage in winding will have a phase which corresponds to the phase of the voltage applied to terminals a and 20b of transformer 2L' If the voltage across resistor 62 is negative, the direct current flowing from to 33 and from 21 to 34 will be decreased, the direct current flowing from 30 to 32 and from 21 to will rem-ainthe same and since the magnetic effect of the directcurrent from 30 to 33 opposes that from 21 to 34 and the magnetic effect of the direct current from 30 to 32 opposes that from 21 to 35 the net result caused thereby will be zero. I

The alternating currents, however, will again not be zero for when the alternating current at terminal 33 is positive, the current'fiow in the direction from 3| to 32 will exceed the current flow in the direction from 21 to 35, and on the remaining half cycle when the alternating current on the terminal 21 ispositive thecurrent and is posiand by the amount by which rmitting'it to retain its sinusoidal characteristics,

It is imperative for the operation of this form of the device that the transformer 29 be constructed with very, close coupling between all of its several windings so that the leakage reactance be held to a minimum and the transfer of energy between its several windings be a maximum. A

flow from terminal flto 35 will exceed the current flow "from 30 to 32 thus'causing an alternating current to now first from terminal 39'to 32 and then from terminal 27 to 35 which will induce a voltage in the secondary 63 of transformer 29 which is determined in magnitude by the degreeto which the grids 53 andfil are made negative and is opposed in phase to the voltage applied to the terminals 23a and 25b of the transformer 2|. u I Thus, at any instant, the magnitude of the modulating voltage E2 determines theoutput voltage across winding 53 of transformer 29 impressing uponit a modulation envelope and yet,

recommended form of its construction is, illustrated in Fig. 3 which is a perspective view of the transformer and in Fig. 4 which is a cross section thereof on a line 44 wherein it is shown that the windings 28 and 3| of the transformer 29 are placed side-by-side onthe center leg 59 of a three-legged core consisting of the laminations 64 and winding 53 fitted so as to enclose the windings 28 and 3 l. The lamination's 54 must have interleaved stacking 65 and the winding must fill as much as possible of the available space in the core.

Other means such as multi-ielernent tubes having multiple grid structures might be used to eliminate the transformer 29 as is apparent to anyone skilled in the art, or other variations may suggest themselvesfor the accomplishment of the same purpose.

What is claimed is:

1. A modulator of the character described-comprising means for separating a carrier frequency voltage of constant amplitude into out of phase components and for superimposing said components of carrier frequency voltage upon a direct current voltage, a plurality of vacuum tubes, transformer means having aplurality of mutually coupled windings to utilize the separated components of carrier frequency voltage and direct current voltage for energizing the anodes of said vacuum tubes in pairs, means to var the conductance of said vacuum tubes for energizing, in pairs,those-grids thereofwhich are not already paired together .with a modulating voltage, the remaining grids being grounded, the second named means co-acting with the variations in conductance of said vacuum tubes to vary the current flow in said vacuum tubes due to said components of carrier frequency voltage and to components and for superimposing said components of carrier frequency voltage upon a direct current voltage, a plurality of vacuum tubes, means to utilize the separated components of carrier frequency voltage and direct current voltage for energizing the anodes of said vacuum tubes in pairs, means to vary the conductance of said vacuum tubes by energizing, inpairs, those grids thereof which are not already paired together with a modulating voltage, the second named means co-acting with said variation in conductance of said vacuum tubes to vary the magnitude of the current flow insaid vacuum tubes due to components of carrier frequency voltage, to direct current and to couple the variations in magnitude due to carrier frequency alone to an output circuit.

3. A modulator of the character described comprising means for separating a carrierfrequency voltage of constant'amplitudeinto out of phase components, a plurality of vacuum tubes,.trans- *ponents of carrier frequency voltagetto energize the anodes of said vacuum tubes in pairs, each pair associated withone winding, means to vary the conductance of said vacuum tubes byenergizing, as a pair, two grids thereofwhicharec not already paired with a modulating Voltage,1the

remaining grids being grounded, the second named transformer means co-acting with said variation in conductance of said vacuum tubes to vary the current flow in said vacuum tubesdue to said components of carrier frequency voltage and to mutually couple: the 'variations thus produced to an output circuit.

4. A modulator of the character described comprising mutually coupled means for separating the carrier frequency voltage of constant amplitude into out of phase components, a plurality of vacuum tubes, means to utilize the components of carrier frequency voltage to energize the anodes of said Vacuum tubes in pairs, means to vary the conductance of said vacuum tubes by energizing, in pairs, those grids thereof which are not already paired together with a modulating voltage, the second named means co-acting of vacuum tubes, a transformer having three or more separate windings, circuit means coupling the first named means to said vacuum tubes to energize the anodes thereof in pairs with the separated components of carrier frequency voltage, means to vary the conductance of said vacuum tubes by energizing those grids thereof which are not already paired together with a modulating voltage, said transformer co-acting with the variation in conductance of said vacuum tubes to vary the magnitude of current flow due to said.

components of said carrier frequency voltage and coupling the variation in magnitude thus produced to an output circuit.

6. A modulator of the character described comprising mutually coupled means for separating the carrier frequency voltage of constant amplitude into out of phase components andfor superimposing said components of said carrier frequency voltage upon a direct current voltage, a

plurality of vacuum tubes, a transformer having three or more separate windingacircuit means coupling the first named means to said vacuum tubes to energize the anodes thereof in pairs with the separated components of carrier frequency voltage, means to vary the conductance of said,

vacuum tubes by energizing those grids thereof which are not already paired togetherwith a modulating voltage, said transformer co-acting with the variation in conductance of said vacuum tubes to vary the magnitude of current flow due to said components of said carrier frequency voltage and coupling thevari-ation in magnitude thus produced to an output circuit.

7. A modulator of the character described,

comprising a pair of vacuum tubes, each having an anode, a cathode, and one or more control electrodes, a pair of impedances, a pair of carrier input terminals and a pair of carrier output terminals, .said vacuum tubes and impedances being arranged, with respectto said carrier in- '1 put ter'minals', so that said vacuuim tubes-are in series; andsaid impedances are in series, and so that said vacuum tubes, in'series, are'in parallel with said impedances in series, the vacuum tubes and-impedances being further arranged so that, with respect' tosaid carrier output' terminals, each of-said vacuum tubesisin series with one of said impedances, and so that one-0f said vacuum tubes=inseries -with one of saidimpedances is in parallel with the other of said vacuum tubes'in series withthe other of said impedances, a source of direct ourrent-inparallel with said carrier :outputte'rminals; meansto prevent the carrier output from'flowing in the direct current source, means for introducing a-modulating voltage, and meansfor varyingthe' conductance of said vacuum tubes "with saidmodulating voltage.

8-." A modulator of the character described, comprising a pair'of'vacuum tubes, each having an anode; cathode, and :one or more control electrodes, a pairof-resistors; a pair of, carrier input terminals and. apair of carrier output terminals, said vacuum-tubes and resistors being arranged, with respect tosaid carrier input terminals, so that said vacuum tubes are in'series, and said resistors are in series," and so.that said vacuum tubes inseries are-in 'para'llel'with said resistors in series, the vacuum tubes and'resisto-rs being furtherarranged. so that, with respect to said carrier output terminals, each of said vacuum tubes is in series withxone of said resistors and in parallel with the other of said vacuum tubes in series With the other of said resistors, a source of direct current, in parallel with said carrier output terminals, means tofprevent .the carrier output from flowing in the direct current source, means for introducing amodulating voltage, and means for varyingthe'conductance .of said vacuum tubes with saidmodulating voltage.

9. A modulator of the character described, comprisingapair. of vacuum tubes eachhaving an anode, c-athode,.and one or-more controlelectrodes, apai-r. of inductors, a pair of carrier input terminals andapair'of carrierw output terminals, said vacuum tubesand inductors being arranged, with. respect to said. carrier input terminals, so that .sa'idvacuum tubes are in series, and said in- .ductors=are. -in series,.and so..that said Vacuum tubes in series. are in parallel'with said inductors -in -series,.,the vacuum.tubes-and inductorsbeing .fuuther arranged so. that, with .respectto said carrier output terminals -eachsof said vacuum tubes-is in series with oneofsaid inductors and so .that one ofsaid vacuum tubes in series with one of said inductors is in parallel with the other of. saidvacuum tubesin series-with the other of said inductors, a source of direct current in parallel with saidcarrier output terminals, means to prevent the carrier output from flowing in the direct current source, means for. introducing amodulating voltage and means for varying the conductance of'sa'id vacuum tubes with saidmodulating voltage.

10.A modulator of the character described, comprisinga pair of vacuum tubes, each having an anode, cathode and one or more control electrodes, and a transformer having two. or more -mutually coupled windings, one of which is center tapped, a pair of carrier input terminals and a pair of carrier output terminals, said vacuum tubes and transformer beingarranged so that with'respect to the carrier input terminals the center tapped winding is inparallel with the aforesaid pair of vacuum tubes Which'are in series, a direct current source, said vacuum tubes and center tapped winding of said transformer being further arranged so that with respect to the direct current source, one-half of said center tapped winding, in series with one of said vacuum tubes, is in parallel with the remaining half ;0f the said center tapped winding, in series with the remaining vacuum tube, means for introducing a modulating voltage, means for varying the conductance of said vacuum tubes in accordance with said modulating voltage and means comprising the remaining, untapped winding of said transformer to keep the carrier output from flowing in the direct current source.

11. A modulator of the character described, comprising a plurality of vacuum tubes and a plurality of impedances which vacuum tubes and impedances are arranged with respect to the input terminals of a carrier voltage so that the impedances are in series, and are in parallel, in pairs, with said vacuum tubes which are in series, said vacuum tubes and impedances being further REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,984,156 Purington Dec. 11, 1934 20 2,120,882 Ballantine June 14, 1938 2,345,712 Mohr Apr. 4, 1944 

